Internal combustion engines, and particularly diesel engines, produce a variety of different particles during combustion. The reason for this is mainly incomplete combustion of the fuel/air mixture and the type of particles produced vary depending on the fuel used and the type of engine. Since soot and other particles from internal combustion engines are generally harmful and contribute to the particulate matter pollution in the air, many jurisdictions apply increasingly strict emission regulations in order to minimize the emissions of particulate matter from internal combustion engines. One way of reducing the emissions is to fit a particulate filter in an exhaust system of the internal combustion engine. Such a filter may efficiently remove particulate matter from the exhaust gas of the engine. However, the filter becomes clogged with soot over time, such that the flow resistance across the filter increases. It is therefore common to regenerate the filter from time to time by increasing the temperature so that clogging soot particles are burnt off.
The necessity to regenerate the particulate filter may for example be determined by studying the flow resistance across the filter. The flow resistance is commonly given by combining the results of a pressure measurement across the filter and an estimation of the volume flow of exhaust gases through the filter. However, the volume flow of exhaust gases across the filter is difficult to estimate accurately due to leakage, temperature variations, evaporation, etc., and large flow variations over time due to e.g. varying throttle actuation and turbulent flows.
DE 4226055 discloses a method of determining a flow resistance across a particulate filter in an exhaust system, in which a first pressure sensing device is used to measure a first differential pressure across the particulate filter, and a second pressure sensing device is used to measure a second differential pressure across a cross-sectional reduction of the exhaust system downstream of the particulate filter. The first and the second differential pressures are used to determine a pressure ratio, which is compared to a threshold value. A filter regeneration is initiated if the difference between the threshold value and the pressure ratio is smaller than a predetermined setpoint.
However, a problem with the method as disclosed in DE 4226055 is that it requires a cross sectional reduction in the exhaust system across which the second differential pressure can be measured. Such a cross sectional reduction increases the flow resistance in the entire exhaust system and is therefore not desirable, since generally a low flow resistance of the system is needed to e.g. determine the flow resistance across the filter. Furthermore, a high flow resistance in the exhaust system may lead to increased fuel consumption.